A voice communication system includes two or more electronic or digital communication devices that are wirelessly or physically coupled to each other. Generally, one of the communication devices includes a transmitter that encodes and packetizes audio data such as speech, and transmits the encoded audio data to a receiver included in a second communications device. At the receiver, packets are received and decoded. Uncorrupted packets are routed directly to an audio output such as a speaker system. Corrupted packets whose access code, header information, or data bits have been garbled during transmission are declared as missing. The corrupted packets create gaps in the reproduced speech, which may be treated as silent intervals or concealed. Treating the gaps as silent intervals requires no signal processing at the receiver. However, the resulting gaps in the reproduced speech are audible and disturbing to the listener.
Alternatively, the gaps in reproduced speech may be covered using packet loss concealment (PLC) techniques. These techniques use various algorithms to generate a synthetic speech signal that has the same timbre and other characteristics as the missing signal. The synthetic speech signal is then inserted into the appropriate gap and blended with speech information that is on either side of the gap to provide reproduced speech that contains no silent intervals.
The PLC technique of waveform substitution examines received packets for waveform segments that resemble the waveforms of the missing packets. When a match or matches occur, the waveform segment(s) are inserted into the gaps to conceal the missing packet. Another technique, known as packet repetition, uses the most recently received packet to generate a reasonable approximation of the missing packet. Advantages of packet repetition are that it requires virtually no signal processing, and that the amount of required speech storage is limited to one packet. A third technique, based on pattern matching, replaces missing packets with packet length segments, extracted from the received speech. A fourth technique estimates the pitch of the received speech and replicates prior pitch waveforms for the duration of the gap. When desirable to maintain phase continuity at the boundaries of substitution packets and prior received packets, the techniques of pitch waveform replication, and pattern matching are preferred over packet repetition.
A significant drawback is that current PLC techniques are limited to pulse code modulation (PCM) coders. Few, if any, PLC techniques have been adapted or developed for continuous variable slope delta modulation (CVSD) coders.